1. Technical Field
The present invention relates to an optical device and a method of manufacturing the same, and more particularly, to an optical device in which a crystal substrate is heated at a high temperature of 1000° C. or more when a dopant is diffused into a stoichiometric lithium niobate crystal, and a method of manufacturing the same.
2. Related Art
Conventionally, in an optical device such as an optical switch or an optical modulator, an optical device using a substrate having an electrooptic effect is provided. As the substrate having an electrooptic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based material, and a combination thereof can be used. In particular, lithium niobate (LiNbO3; hereinafter, referred to as LN) crystal having a high electrooptic effect is preferably used.
A CLN crystal substrate which is a LN crystal substrate having congruent melting composition in which crystal and melt co-exist with the same composition in equilibrium is used as the substrate used in the optical device. In the CLN crystal, the mole fraction of Li is 48.5 mol % with respect to Li and Nb.
On the other hand, the LN crystal substrate in which the mole composition ratio of Li to Nb is at least 48.5 mol %, more preferably, 49.5 to 50.5 mol %, and a Curie temperature measured using a differential thermal analysis (DTA) or a differential scanning calorimetry (DSC) is about 1200° C., which is higher than 1150° C. of a general CLN, is called a stoichiometric or SLN crystal substrate (hereinafter, referred to as SLN). Since the SLN crystal has higher electrooptic effect than that of the CLN crystal, as disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2003-177258 is an example of related art), when the SLN crystal is used as an optical device, a device length is short and an operation voltage is low and thus it is possible to provide an optical device having excellent integration. In addition, when an optical waveguide is formed in the SLN crystal, confinement of light is strong, and thus, when the SLN crystal is used in the optical modulator, a structure having high efficiency in refractive index change due to applying an electric field can be designed.
In general, when the LN crystal is heated at a high temperature of, for example, 650° C. or more, out-diffusion of Li2O constituting LN occurs. In a region in which the out-diffusion occurs, a refractive index of extraordinary light increases (Δne/ΔC(mol %)=−0.016. Here, Δne denotes a change in the refractive index and ΔC denotes a change in the mole fraction of Li).
An optical device, and more particularly, a device using a refractive index difference between a dopant diffusion portion and a non-diffusion portion needs to be designed for anticipating increase of the refractive index of the diffusion portion and the change in the refractive index of the non-diffusion portion and thus manufacture reproducibility is difficult. In addition, in a case of a waveguide type device, a whole or partial slab waveguide occurs and confinement of light wave in the waveguide becomes weaker due to the increase of the surface refractive index.
It is effective to increase a vapor pressure of Li2O during the heat treatment in order to suppress the out-diffusion of Li2O, and a method of simultaneously introducing powdered Li2O and LN into a heating device or a method of sealing the LN crystal with a material (for example, platinum or high-purity quartz) which does not react with LN even under a high temperature is known. In addition, a method of performing the heat treatment in an oxygen atmosphere containing water vapor, that is, a wet atmosphere, to suppress the out-diffusion and to obtain a high-quality waveguide having low propagation loss is disclosed in Non-patent Document 1 (Hiroshi Nishihara et al., “Optical integrated circuit”, revised and enlarged edition, Ohmsha, Ltd., Jul. 25, 1999, p 172-174) and Non-patent Document 2 (T. Nozawa, H. Miyazawa and S. Miyazawa: “Water vapor effects on titanium diffusion into LiNbO3 substrates”, Jpm. J. Appl. Phys., vol. 29, No 10, pp 2180-2185). Since control of a water vapor pressure is simpler than control of the vapor pressure of Li2O, the heat treatment in the wet atmosphere is generally employed in manufacturing the LN optical waveguide.
When an optical waveguide is formed on a SLN crystal substrate by thermally diffusing Ti, as disclosed in Patent Document 1 or Non-patent Document 3, a diffusion coefficient of the SLN crystal is smaller than that of the CLN crystal. In Patent Document 1, a Ti film of 70 nm is formed on the surface of the SLN crystal substrate by electron beam evaporation and thermally diffused at a diffusion temperature of 1000° C. to 1060° C. for a diffusion time of 6 to 24 hours in an oxygen atmosphere containing water vapor. For example, when the diffusion is performed at a diffusion temperature of 1030° C. for 10 hours, an optical waveguide having a depth of 1.6 μm is formed in the Z-plate SLN crystal substrate.
In Non-patent Document 3 (Takahiro Oka et al., “Wavelength dependency of a Ti diffusion waveguide in stoichiometric lithium niobate crystal”, proceedings of 63th applied physics association lecture meeting, p. 1045, September, 2002), when a deposited Ti film (190 nm) is formed in the SLN crystal and diffused at a diffusion temperature of 1060° C. for a diffusion time of 48 to 192 hours in a wet-O2 atmosphere, an optical waveguide having a depth of about 5 μm is formed.
When the optical waveguide is formed using the SLN crystal substrate as described above, the diffusion temperature or the diffusion time when diffusing a dopant into the substrate needs to become greatly higher or longer than those of the CLN crystal substrate.
However, even in the SLN crystal, the out-diffusion of Li2O occurs upon the heat treatment of a high temperature, similar to the CLN crystal. When the SLN substrate is used, since the temperature of the heat treatment required for manufacturing the optical waveguide is high or the time thereof is long, the change in the refractive index due to the out-diffusion of Li2O more increases. Since the refractive index of the whole substrate surface increases depending on the temperature or the time, the whole substrate surface may become the slab waveguide. When the dopant is thermally diffused or when the heat treatment is performed in order to compensate for process distortion by heating at a high temperature, the refractive index of the substrate surface needs to be suppressed from increasing.
A relationship between the heating time of the CLN or SLN and the increase of the refractive index is disclosed in Non-Patent Document 4 (Hiroshi Yamauchi, et al., “Influence of out-diffusion of Li2O in stoichiometric lithium niobate crystal in a diffusion atmosphere”, proceedings of 50th applied physics association lecture meeting, p. 1258, 2003). In addition, the “relationship between the heating time and the change in the refractive index of extraordinary light at a diffusion temperature of 1030° C. in a wet-O2 atmosphere” disclosed in Non-patent document 4 is shown in Table 1.
TABLE 1Relationship between heating time and change inrefractive index of extraordinary lightChange in refractive index ofextraordinary light (Δne)45 h75 h120 hCLN (condition A)1 × 10−4 or less1 × 10−4 or less5 × 10−4SLN (condition A)3 × 10−43 × 10−45 × 10−4CLN (condition B)1 × 10−32 × 10−33 × 10−3SLN (condition B)1 × 10−32 × 10−33 × 10−3Condition A: sealing into Pt-boxCondition B: No sealing
In addition, Mg:SLN crystal in which Mg is doped into the SLN crystal has higher electrooptic effect and lower nonstoichiometric defect and is stronger against optical damage, compared with the SLN crystal. However, the diffusion coefficient of the dopant of the Mg:SLN crystal is lower than that of the SLN crystal. Accordingly, in the heat treatment for thermally diffusing the dopant into the Mg:SLN crystal, a heating temperature and a heating time more increase than those of the SLN crystal. Thus, the above-mentioned problems become serious and a manufacturing cost increases.
The invention provides a optical device having high quality, excellent productivity and optical characteristics, and capable of suppressing a refractive index of a substrate surface from increasing when a dopant is thermally diffused into, or a heat treatment is performed in order to compensate for process distortion in stoichiometric lithium niobate crystal or a crystal substrate in which Mg is doped into the crystal, and a method of manufacturing the optical device.